roles/kvm: fix option description eval

`literalExpression` was not taken from lib and hence caused the
fc-search eval to fail.

I also checked all other occurances of `literalExpressions` in the
platform code. Almost all of them ar placed after a global `with lib;`
which is bad style but at least causes them to eval fine.

PL-133423

Author: osnyx

nixos/roles/kvm.nix

@@ -35,7 +35,7 @@ in
           Can be replaced for development and testing purposes.
         '';
         default = cephPkgs.fc-qemu;
-        defaultText = literalExpression "pkgs.fc.qemu [parameterised with cephRelease]";
+        defaultText = lib.literalMD "`pkgs.fc.qemu` *[parameterised with cephRelease]*";
       };
       cephRelease = fclib.ceph.releaseOption // {
         description = "Codename of the Ceph release series used by qemu.";